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United States Patent |
5,538,816
|
Hashimoto
,   et al.
|
July 23, 1996
|
Halftone phase shift photomask, halftone phase shift photomask blank,
and methods of producing the same
Abstract
A halftone phase shift photomask designed so that it is possible to shorten
the photoengraving process, use a production line for a conventional
photomask, prevent lowering of the contract between the transparent and
semitransparent regions at a long wavelength in the visible region, which
is used for inspection and measurement, and also prevent charge-up during
electron beam exposure, and that ordinary physical cleaning process can be
used for the halftone phase shift photomask. The halftone phase shift
photomask has on a transparent substrate (1) a region which is
semitransparent to exposure light and a region which is transparent to the
exposure light so that the phase difference between light passing through
the transparent region and light passing through the semitransparent
region is substantially .pi. radians. A semitransparent film that
constitutes the semitransparent region is arranged in the form of a
multilayer film including layers (3, 4) of chromium or a chromium
compound. For example, the layer (3) is formed of chromium oxide, chromium
oxide nitride, chromium oxide carbide, or chromium oxide nitride carbide,
and the layer (4) is formed of chromium or chromium nitride. The layer (3)
mainly serves as a phase shift layer, while the layer (4) mainly serves as
a transmittance control layer that suppresses the rise of transmittance at
the long wavelength side. The semitransparent film is formed by physical
vapor deposition.
Inventors:
|
Hashimoto; Keiji (Tokyo, JP);
Fujikawa; Junji (Tokyo, JP);
Mohri; Hiroshi (Tokyo, JP);
Takahashi; Masahiro (Tokyo, JP);
Miyashita; Hiroyuki (Tokyo, JP);
Iimura; Yukio (Tokyo, JP)
|
Assignee:
|
Dai Nippon Printing Co., Ltd. (Tokyo, JP);
Mitsubishi Electric Corporation (Tokyo, JP)
|
Appl. No.:
|
225905 |
Filed:
|
April 11, 1994 |
Foreign Application Priority Data
| Apr 09, 1993[JP] | 5-083433 |
| Apr 09, 1993[JP] | 5-083434 |
| Jul 13, 1993[JP] | 5-173042 |
Current U.S. Class: |
430/5; 427/582; 428/203; 428/472; 428/696; 428/698; 430/322 |
Intern'l Class: |
G03F 009/00 |
Field of Search: |
430/5,322
428/203,472,696,698
427/582
|
References Cited
U.S. Patent Documents
5230971 | Jul., 1993 | Alpay | 430/5.
|
5286581 | Feb., 1994 | Lee | 430/5.
|
Primary Examiner: Rosasco; S.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak & Seas
Claims
What we claim is:
1. A halftone phase shift photomask having on a transparent substrate a
halftone phase shift layer which includes at least one layer composed
mainly of a chromium compound,
wherein the ratio of the number of chromium atoms to the number of oxygen
atoms in said layer composed mainly of a chromium compound, when measured
by X-ray photoelectron spectroscopy, falls within the range of from
100:100 to 100:300.
2. A halftone phase shift photomask according to claim 1, wherein said at
least one layer composed mainly of a chromium compound contains a number
of carbon atoms which is not smaller than 2% of the number of chromium
atoms.
3. A halftone phase shift photomask according to claim 2, wherein a larger
number of carbon atoms are contained in a surface region within the depth
of 3 nm from the surface of said at least one layer composed mainly of a
chromium compound than in the other region.
4. A halftone phase shift photomask according to claim 1, wherein said at
least one layer composed mainly of a chromium compound contains nitrogen
atoms in such a proportion that the total number of nitrogen and oxygen
atoms is not larger than 350 per 100 chromium atoms.
5. A halftone phase shift photomask according to claim 1, wherein said at
least one layer composed mainly of a chromium compound contains argon
atoms in such a proportion that the total number of argon and oxygen atoms
is not larger than 350 per 100 chromium atoms.
6. A halftone phase shift photomask according to any of claims 1 to 5,
wherein said at least one layer composed mainly of a chromium compound
contains impurity atoms other than chromium, oxygen, carbon, nitrogen and
argon atoms within the range in which the refractive index for a
wavelength of exposure light that is obtained by ellipsometry will not be
changed by 0.1 or more.
7. A halftone phase shift photomask according to any of claims 1 to 6,
wherein said halftone phase shift layer is formed on said transparent
substrate so that the phase difference .phi., which is obtained by the
following equation, is n.pi..+-..pi./3 radians (n is an odd integer):
##EQU8##
where .phi. is the phase change of the light perpendicularly passing
through the photomask having a multilayer (m-2 layers) film formed on said
substrate, x.sup.k,k+1 is the phase change occurring at the interface
between the k-th layer and the (k+1)th layer, u.sub.k and d.sub.k are the
refractive index of a material constituting the k-th layer and the
thickness of the k-th layer, respectively, and .lambda. is the wavelength
of exposure light, and where the layer of k=1 is assumed to be said
transparent substrate, and the layer of k=m is assumed to be air.
8. A halftone phase shift photomask according to any of claims 1 to 7,
wherein said halftone phase shift layer is formed on said transparent
substrate at such a thickness that the transmittance for exposure light is
in the range of from 1% to 50%.
9. A halftone phase shift photomask blank having on a transparent substrate
a halftone phase shift layer which includes at least one layer composed
mainly of a chromium compound,
wherein the ratio of the number of chromium atoms to the number of oxygen
atoms in said layer composed mainly of a chromium compound, when measured
by X-ray photoelectron spectroscopy, falls within the range of from
100:100 to 100:300.
10. A halftone phase shift photomask blank according to claim 9, wherein
said at least one layer composed mainly of a chromium compound contains a
number of carbon atoms which is not smaller than 2% of the number of
chromium atoms.
11. A halftone phase shift photomask blank according to claim 10, wherein a
larger number of carbon atoms are contained in a surface region within the
depth of 3 nm from the surface of said at least one layer composed mainly
of a chromium compound than in the other region.
12. A halftone phase shift photomask blank according to claim 9, wherein
said at least one layer composed mainly of a chromium compound contains
nitrogen atoms in such a proportion that the total number of nitrogen and
oxygen atoms is not larger than 350 per 100 chromium atoms.
13. A halftone phase shift photomask blank according to claim 9, wherein
said at least one layer composed mainly of a chromium compound contains
argon atoms in such a proportion that the total number of argon and oxygen
atoms is not larger than 350 per 100 chromium atoms.
14. A halftone phase shift photomask blank according to any of claims 9 to
13, wherein said at least one layer composed mainly of a chromium compound
contains impurity atoms other than chromium, oxygen, carbon, nitrogen and
argon atoms within the range in which the refractive index for a
wavelength of exposure light that is obtained by ellipsometry will not be
changed by 0.1 or more.
15. A halftone phase shift photomask blank according to any of claims 9 to
14, wherein said halftone phase shift layer is formed on said transparent
substrate so that the phase difference .phi., which is obtained by the
following equation, is n.pi..+-..pi./3 radians (n is an odd integer):
##EQU9##
where .phi. is the phase change of the light perpendicularly passing
through the photomask having a multilayer (m-2 layers) film formed on said
substrate, x.sup.k,k+1 is the phase change occurring at the interface
between the k-th layer and the (k+1)th layer, u.sub.k and d.sub.k are the
refractive index of a material constituting the k-th layer and the
thickness of the k-th layer, respectively, and .lambda. is the wavelength
of exposure light, and where the layer of k=1 is assumed to be said
transparent substrate, and the layer of k=m is assumed to be air.
16. A halftone phase shift photomask blank according to any of claims 9 to
15, wherein said halftone phase shift layer is formed on said transparent
substrate at such a thickness that the transmittance for exposure light is
in the range of from 1% to 50%.
17. A halftone phase shift photomask having on a transparent substrate a
region which is semitransparent to exposure light and a region which is
transparent to the exposure light so that the phase difference between
light passing through said transparent region and light passing through
said semitransparent region is substantially .pi. radians,
wherein said semitransparent region is formed from a multilayer
semitransparent film of chromium or a chromium compound.
18. A halftone phase shift photomask according to claim 17, wherein said
semitransparent film is arranged in the form of a double-layer film which
includes, in order from the transparent substrate side, a single-layer
film of a compound selected from among chromium oxide, chromium oxide
nitride, chromium oxide carbide, and chromium oxide nitride carbide, and a
single-layer film of either chromium or chromium nitride.
19. A halftone phase shift photomask according to claim 17, wherein said
semitransparent film is arranged in the form of a double-layer film which
includes, in order from the transparent substrate side, a single-layer
film of either chromium or chromium nitride, and a single-layer film of a
compound selected from among chromium oxide, chromium oxide nitride,
chromium oxide carbide, and chromium oxide nitride carbide.
20. A halftone phase shift photomask according to claim 17, wherein said
semitransparent film is arranged in the form of a triple-layer film which
includes, in order from the transparent substrate side, a single-layer
film of a compound selected from among chromium oxide, chromium oxide
nitride, chromium oxide carbide, and chromium oxide nitride carbide, a
single-layer film of either chromium or chromium nitride, and a
single-layer film of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide.
21. A halftone phase shift photomask according to claim 17, wherein said
semitransparent film is arranged in the form of a double-layer film which
includes, in order from the transparent substrate side, a single-layer
film of an electrically conductive chromium compound, and a single-layer
film of a compound selected from among chromium oxide, chromium oxide
nitride, chromium oxide carbide, and chromium oxide nitride carbide.
22. A halftone phase shift photomask according to claim 17, wherein said
semitransparent film is arranged in the form of a double-layer film which
includes, in order from the transparent substrate side, a single-layer
film of a compound selected from among chromium oxide, chromium oxide
nitride, chromium oxide carbide, and chromium oxide nitride carbide, and a
single-layer film of an electrically conductive chromium compound.
23. A halftone phase shift photomask according to claim 17, wherein said
semitransparent film is arranged in the form of a triple-layer film which
includes, in order from the transparent substrate side, a single-layer
film of a compound selected from among chromium oxide, chromium oxide
nitride, chromium oxide carbide, and chromium oxide nitride carbide, a
single-layer film of an electrically conductive chromium compound, and a
single-layer film of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide.
24. A halftone phase shift photomask according to any of claims 17 to 23,
wherein said semitransparent region is formed on said transparent
substrate so that the phase difference .PHI. between said semitransparent
region and said transparent region, which is obtained by the following
equation, falls within the range of n.pi..+-..pi./3 radians (n is an odd
integer):
##EQU10##
where .PHI. is the phase difference of the light perpendicularly passing
through the photomask having a multilayer (m-2 layers) film formed on said
transparent substrate, n.sub.k, d.sub.k are the refractive index and
thickness, respectively, of a material constituting the k-th layer, and
.lambda. is the wavelength of exposure light, and where the layer of k=1
is assumed to be said transparent substrate, and the layer of k=m (m>3; m
is an integer) is assumed to be air.
25. A halftone phase shift photomask according to any of claims 17 to 24,
wherein said semitransparent region is formed so that its transmittance
for exposure light is in the range of from 1% to 50%.
26. A method of producing a halftone phase shift photomask having on a
transparent substrate a region which is semitransparent to exposure light
and a region which is transparent to the exposure light so that the phase
difference between light passing through said transparent region and light
passing through said semitransparent region is substantially .pi. radians,
said semitransparent region being formed from a triple-layer transparent
film which includes, in order from the transparent substrate side, a
single-layer film of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide, a single-layer film of an electrically conductive chromium
compound, and a single-layer film of a compound selected from among
chromium oxide, chromium oxide nitride, chromium oxide carbide, and
chromium oxide nitride carbide,
wherein a transmittance of said semitransparent region is controlled by
varying the thickness of either or each of said first and third
single-layer films, which are made of a compound selected from among
chromium oxide, chromium oxide nitride, chromium oxide carbide, and
chromium oxide nitride carbide, and the thickness of said single-layer
film of an electrically conductive chromium compound.
27. A method of producing a halftone phase shift photomask having on a
transparent substrate a region which is semitransparent to exposure light
and a region which is transparent to the exposure light so that the phase
difference between light passing through said transparent region and light
passing through said semitransparent region is substantially .pi. radians,
said semitransparent region being formed from a triple-layer transparent
film which includes, in order from the transparent substrate side, a
single-layer film of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide, a single-layer film of an electrically conductive chromium
compound, and a single-layer film of a compound selected from among
chromium oxide, chromium oxide nitride, chromium oxide carbide, and
chromium oxide nitride carbide,
wherein a reflectivity of the obverse or reverse surface of said mask is
controlled by varying the thickness ratio between said first and third
single-layer films, which are made of a compound selected from among
chromium oxide, chromium oxide nitride, chromium oxide carbide, and
chromium oxide nitride carbide.
28. A halftone phase shift photomask blank having on a transparent
substrate a region which is semitransparent to exposure light and a region
which is transparent to the exposure light so that the phase difference
between light passing through said transparent region and light passing
through said semitransparent region is substantially .pi. radians,
wherein said semitransparent region is formed from a multilayer
semitransparent film of chromium or a chromium compound.
29. A halftone phase shift photomask blank according to claim 28, wherein
said semitransparent film is arranged in the form of a double-layer film
which includes, in order from the transparent substrate side, a
single-layer film of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide, and a single-layer film of either chromium or chromium nitride.
30. A halftone phase shift photomask blank according to claim 28, wherein
said semitransparent film is arranged in the form of a double-layer film
which includes, in order from the transparent substrate side, a
single-layer film of either chromium or chromium nitride, and a
single-layer film of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide.
31. A halftone phase shift photomask blank according to claim 28, wherein
said semitransparent film is arranged in the form of a triple-layer film
which includes, in order from the transparent substrate side, a
single-layer film of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide, a single-layer film of either chromium or chromium nitride, and a
single-layer film of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide.
32. A halftone phase shift photomask blank according to claim 28, wherein
said semitransparent film is arranged in the form of a double-layer film
which includes, in order from the transparent substrate side, a
single-layer film of an electrically conductive chromium compound, and a
single-layer film of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide.
33. A halftone phase shift photomask blank according to claim 28, wherein
said semitransparent film is arranged in the form of a double-layer film
which includes, in order from the transparent substrate side, a
single-layer film of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide, and a single-layer film of an electrically conductive chromium
compound.
34. A halftone phase shift photomask blank according to claim 28, wherein
said semitransparent film is arranged in the form of a triple-layer film
which includes, in order from the transparent substrate side, a
single-layer film of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide, a single-layer film of an electrically conductive chromium
compound, and a single-layer film of a compound selected from among
chromium oxide, chromium oxide nitride, chromium oxide carbide, and
chromium oxide nitride carbide.
35. A halftone phase shift photomask blank according to any of claims 28 to
34, wherein said semitransparent region is formed on said transparent
substrate so that the phase difference .PHI. between said semitransparent
region and said transparent region, which is obtained by the following
equation, falls within the range of n.pi..+-..pi./3 radians (n is an odd
integer):
##EQU11##
where .PHI. is the phase difference of light perpendicularly passing
through the photomask having a multilayer (m-2 layers) film formed on said
transparent substrate, n.sub.k, d.sub.k are the refractive index and
thickness, respectively, of a material constituting the k-th layer, and
.lambda. is the wavelength of exposure light, and where the layer of k=1
is assumed to be said transparent substrate, and the layer of k=m (m>3; m
is an integer) is assumed to be air.
36. A halftone phase shift photomask blank according to any of claims 28 to
35, wherein said semitransparent region is formed so that its
transmittance for exposure light is in the range of from 1% to 50%.
37. A method of producing a blank used to produce a halftone phase shift
photomask having on a transparent substrate a region which is
semitransparent to exposure light and a region which is transparent to the
exposure light so that the phase difference between light passing through
said transparent region and light passing through said semitransparent
region is substantially .pi. radians, said semitransparent region being
formed from a triple-layer transparent film which includes, in order from
the transparent substrate side, a single-layer film of a compound selected
from among chromium oxide, chromium oxide nitride, chromium oxide carbide,
and chromium oxide nitride carbide, a single-layer film of an electrically
conductive chromium compound, and a single-layer film of a compound
selected from among chromium oxide, chromium oxide nitride, chromium oxide
carbide, and chromium oxide nitride carbide,
wherein a transmittance of said semitransparent region is controlled by
varying the thickness of either or each of said first and third
single-layer films, which are made of a compound selected from among
chromium oxide, chromium oxide nitride, chromium oxide carbide, and
chromium oxide nitride carbide, and the thickness of said single-layer
film of an electrically conductive chromium compound.
38. A method of producing a blank used to produce a halftone phase shift
photomask having on a transparent substrate a region which is
semitransparent to exposure light and a region which is transparent to the
exposure light so that the phase difference between light passing through
said transparent region and light passing through said semitransparent
region is substantially .pi. radians, said semitransparent region being
formed from a triple-layer transparent film which includes, in order from
the transparent substrate side, a single-layer film of a compound selected
from among chromium oxide, chromium oxide nitride, chromium oxide carbide,
and chromium oxide nitride carbide, a single-layer film of an electrically
conductive chromium compound, and a single-layer film of a compound
selected from among chromium oxide, chromium oxide nitride, chromium oxide
carbide, and chromium oxide nitride carbide,
wherein a reflectivity of the obverse or reverse surface of said mask is
controlled by varying the thickness ratio between said first and third
single-layer films, which are made of a compound selected from among
chromium oxide, chromium oxide nitride, chromium oxide carbide, and
chromium oxide nitride carbide.
39. A halftone phase shift photomask having on a transparent substrate a
transparent region and a semitransparent region which is defined by a
semitransparent layer of a chromium compound formed by physical vapor
deposition, wherein when the transmittance of said transparent region for
a wavelength of exposure light applied during transfer is assumed to be
100%, the transmittance of said semitransparent region falls within the
range of from 3% to 35%, and said semitransparent region shifts the phase
of the exposure wavelength through substantially .pi. radians with respect
to said transparent region.
40. A halftone phase shift photomask according to claim 39, wherein said
chromium compound consists essentially of chromium and oxygen, or
chromium, oxygen and nitrogen, or chromium, oxygen and carbon, or
chromium, oxygen, nitrogen and carbon, and said semitransparent layer is
formed from a single-layer film or a multilayer film including two or more
layers.
41. A halftone phase shift photomask blank having on a transparent
substrate a semitransparent layer of a chromium compound formed by
physical vapor deposition, wherein when the transmittance of said
transparent substrate for a wavelength of exposure light applied during
transfer is assumed to be 100%, the transmittance of said semitransparent
layer falls within the range of from 3% to 35%, and said semitransparent
layer shifts the phase of the exposure wavelength through substantially
.pi. radians.
42. A halftone phase shift photomask blank according to claim 41, wherein
said chromium compound consists essentially of chromium and oxygen, or
chromium, oxygen and nitrogen, or chromium, oxygen and carbon, or
chromium, oxygen, nitrogen and carbon, and said semitransparent layer is
formed from a single-layer film or a multilayer film including two or more
layers.
Description
BACKGROUND OF THE INVENTION
The present invention relates to photomasks used for producing integrated
circuits of high integration density, e.g., large-scale integrated
circuits (LSI), very large-scale integrated circuits (VLSI), etc., and to
photomask blanks used to produce such photomasks. More particularly, the
present invention relates to a halftone phase shift photomask whereby a
projected image of very small size can be obtained, and also to a halftone
phase shift photomask blank for producing the halftone phase shift
photomask. Further, the present invention relates to methods of producing
the halftone phase shift photomask and the halftone phase shift photomask
blank.
Semiconductor integrated circuits, e.g., IC, LSI, VLSI, etc., are produced
by repeating thin film forming processes, e.g., oxidation, CVD or
sputtering, a photolithography process and a diffusion process, e.g., ion
implantation. In the photolithography process, a resist is coated on a
substrate to be processed, e.g., a silicon wafer, and this resist is
subjected to exposure by a reduction projection stepper or other exposure
systems using a photomask to form a desired pattern thereon, followed by
development and etching.
With the achievement of high-operating speed and high integration of
semiconductor integrated circuits, the minimum size of photoresist
patterns formed by the above-described photolithography process has
increasingly been demanded to become smaller. Accordingly, the demanded
device patterns cannot be realized by the conventional reduction
projection stepper exposure method that employs an ordinary photomask
because of the resist pattern resolution limit of this method. To overpass
this limit, a phase shift photomask having a novel structure and a phase
shift exposure method that uses the phase shift photomask have been
proposed, as disclosed, for example, in Japanese Patent Application
Laid-Open (KOKAI) No. 58-173744 (1983) and Japanese Patent Application
Post-Exam Publication No. 62-59296 (1987). The phase shift exposure method
is a technique whereby the resolution and the depth of focus are improved
by controlling the phase of exposure light passing through a phase shift
pattern formed on a photomask.
Phase shift photomasks having various arrangements have been proposed.
Among them, what is called halftone phase shift photomask such as those
disclosed in U.S. Pat. No. 4,890,309 and Japanese Patent Application
Laid-Open (KOKAI) No. 4-136854 (1992) has attracted attention from the
expectation that it will soon be put to practical use, and some proposals
have been made with regard to arrangements and materials of the halftone
phase shift photomask, which enable an improvement in yield and a
reduction in cost as a result of a reduction in the number of
manufacturing steps required. For example, see Japanese Patent Application
Laid-Open (KOKAI) Nos. 5-2259 (1993) and 5-127361 (1993).
The halftone phase shift photomask will briefly be explained below with
reference to the accompanying drawings. FIG. 3 shows the principle of the
halftone phase shift lithography, and FIG. 4 shows a conventional
lithography method. FIGS. 3(a) and 4(a) are sectional views showing
photomasks. FIGS. 3(b) and 4(b) each show the amplitude of light on the
photomask. FIGS. 3(c) and 4(c) each show the amplitude of light on a
wafer. FIGS. 3(d) and 4(d) each show the light intensity on the wafer.
Reference numerals 101 and 201 denote substrates, and 202 a 100%
light-shielding film. A semitransparent film 102 shifts the phase of
incident light through substantially 180.degree. and has a transmittance
of 1% to 50%. Reference numerals 103 and 203 denote incident light. In the
conventional method, as shown in FIG. 4(a), the 100% light-shielding film
202, which is made of chromium, for example, is formed on the substrate
201, which is made of quartz (fused silica), for example, and the
light-shielding film 202 is merely formed with a light-transmitting
portion in a desired pattern. Accordingly, the light intensity
distribution on the wafer has a gentle slope, as shown in FIG. 4(d). As a
result, the resolution is degraded. In the halftone phase shift
lithography, on the other hand, the light passing through the
semitransparent film 102 and the light passing through the opening in the
film 102 are in substantially inverse relation to each other in terms of
phase. Accordingly, the light intensity at the pattern boundary portion on
the wafer is 0, as shown in FIG. 3(d). Thus, it is possible to prevent the
light intensity distribution from exhibiting a gentle slope. Accordingly,
the resolution can be improved.
The halftone phase shift photomask has on a transparent substrate at least
a region which is semitransparent to exposure light and a region which is
transparent to the exposure light so that the phase difference between the
light passing through the semitransparent region and the light passing
through the transparent region is substantially .pi. radians. With the
halftone phase shift photomask, the resolution improves and the depth of
focus enlarges at holes, dots, spaces, lines, etc. on semiconductor
devices. The halftone phase shift photomask is most effective when the
following relation is satisfied:
d=.lambda./{2(n-1)}
where d is the thickness of the semitransparent film, .lambda. is the
wavelength of exposure light, and n is the refractive index of the
semitransparent film for the wavelength of exposure light.
It should be noted here that a phase shift photomask of a type other than
halftone phase shift photomask requires at least two photoengraving
processes to produce a mask pattern because the light-shielding film and
the phase shifter film have different patterns, whereas the halftone phase
shift photomask essentially requires only one photoengraving process
because it has only one pattern; this is a great advantage of the halftone
phase shift photomask.
Incidentally, the semitransparent film 102 of the halftone phase shift
photomask is demanded to perform two functions, that is, phase inversion
and transmittance control. To realize these two functions, the
semitransparent film 102 may be arranged either in the form of a
single-layer film wherein a single layer takes charge of both functions or
in the form of a multilayer film wherein the two functions are assigned to
respective layers. In the former case, the photoengraving process is
required only once. Therefore, it is possible to make use of the advantage
of the halftone phase shift photomask. In the latter case, however, two
photoengraving processes must be carried out because the materials of
constituent layers are different even in a case where the identical
pattern is formed, as will be clear by taking a look at an arrangement in
which a phase shifter layer of spin-on-glass (SOG) is used as a layer that
effects phase inversion, and a chromium light-shielding layer is used as a
layer that performs transmittance control. Therefore, the multilayer film
arrangement results in a rise in cost and a reduction in yield.
Accordingly, it has heretofore been necessary to form the semitransparent
film 102 in a single-layer structure. Even when it adopts a multilayer
structure, it is necessary to find a combination of a phase shifter layer
material and a light-shielding layer material which enables the
semitransparent film 102 to be formed by a single photoengraving process.
However, there is known no semitransparent film material that satisfies the
above-described requirements and that can be used without any substantial
problems from the viewpoint of the photomask producing process. Thus, it
is extremely difficult to select a semitransparent film material. A film
which is composed mainly of a chromium compound, proposed in Japanese
Patent Application Laid-Open (KOKAI) No. 5-127361 (1993), is the only
example that may satisfy the above-described requirements. However,
chromium compounds largely vary in optical characteristics according to
the chemical composition thereof. Therefore, in many cases, chromium
compounds cannot practically be used as a semitransparent film for a
halftone phase shift photomask.
In view of the above-described circumstances, it is a first object of the
present invention to provide a halftone phase shift photomask which has a
simple structure and enables the photoengraving process to be shortened,
thereby making it possible to attain a reduction in cost and an
improvement in yield, and which also enables the greater part of a
production line for a conventional chromium photomask to be used as it is,
and also provide a halftone phase shift photomask blank for producing the
halftone phase shift photomask.
In the meantime, a halftone phase shift photomask structure such as that
shown in FIG. 17 has been proposed. In the figure, reference numeral 501
denotes a quartz substrate, and 502 a chromium thin film. The chromium
thin film 502 forms a semitransparent region, and a region where no
chromium thin film 502 is present defines a transparent region. The mask
with such a structure has the problem that it is difficult to repair a
defect, although the mask can be processed, inspected and cleaned in a
similar manner to the conventional process.
As a structure wherein a defect can be repaired in the conventional
repairing process, a single-layer film of a metal oxide may be considered.
FIG. 18 shows the transmittance spectrum in the wavelength range of 200 nm
to 800 nm of chromium oxide as one example of metal oxides. The thickness
d of this film has been adjusted so as to satisfy d=.lambda./{2(n-1)} for
the exposure wavelength of the i-line (365 nm) of a super-high pressure
mercury lamp. As will be clear from FIG. 18, the transmittance for this
exposure wavelength is sufficiently low, but the transmittance for long
wavelengths in the visible region, which are used for inspection,
measurement, etc., is high.
Thus, in a phase shifter comprising a single layer of a metal oxide, it is
possible to control the transmittance for the exposure wavelength, but the
transmittance undesirably rises at the long wavelength side. Wavelengths
at the long wavelength side, particularly the e-line (546 nm) of a
super-high pressure mercury lamp, are used for inspection of photomasks
and size measurement thereof. However, in the inspection of a mask having
a single-layer phase shifter of a metal oxide, if the transmittance for
the e-line exceeds 30%, inspection and size measurement cannot be
performed because of a reduction in contrast between the transparent and
semitransparent regions.
Further, since such a metal oxide has no electric conductivity, charge-up
occurs during electron beam exposure, causing displacement of the resist
pattern.
In addition, the transmittance of halftone phase shift photomasks varies
according to exposure conditions and device manufacturers. Therefore,
various levels of transmittance are demanded. It is difficult to control
the refractive index and the extinction coefficient so that only the
transmittance varies according to film forming conditions without changing
the phase difference.
In view of the above-described problems of the background art, it is a
second object of the present invention to provide a halftone phase shift
photomask which is designed so that:
1 the rise in the transmittance for a long wavelength in the visible
region, which is used for inspection, measurement, etc., is suppressed to
prevent reduction in contrast between the transparent and semitransparent
regions, thereby facilitating inspection and measurement;
2 electric conductivity is imparted to the phase shifter film to thereby
prevent occurrence of charge-up;
3 the control of transmittance can be facilitated with the exposure
wavelength phase difference held at 180.degree.;
4 the reflectivities of the obverse and reverse surfaces can be controlled;
and
5 an optimal multilayer structure can be realized for each of at least two
different kinds of exposure light by controlling the thickness of each
layer with the film composition of each layer maintained as it is.
FIG. 23 is a sectional view showing one example of a halftone phase shift
photomask disclosed in Japanese Patent Application Laid-Open (KOKAI) No.
4-136854 (1992) as a conventional example of the halftone phase shift
photomask relating to the present invention. The conventional photomask
includes a glass substrate 411 and a patterned semitransparent film 412
provided thereon. The semitransparent film 412 is made of a spin-on-glass
(SOG) having a light-absorbing material added thereto.
However, SOG generally has a low bond strength with respect to the
substrate in comparison to a film that is formed by physical vapor
deposition (PVD), e.g., sputtering, which is used for ordinary photomasks.
Accordingly, the film may be separated or cracked during a physical
cleaning process which is commonly carried out in ordinary photomask
processing by using a brush scrubber, a high-pressure jet spray, an
ultrasonic cleaner, etc. Therefore, it is difficult to clean the film
satisfactorily.
Further, the refractive index of SOG for the exposure wavelength of 365 nm
(i-line) is generally low, i.e., of the order of 1.4 to 1.5, and it is
necessary in order to effect a 180.degree. phase shift to provide SOG to a
thickness of about 365 nm to 456 nm, which is larger than the thickness of
light-shielding films, composed mainly of chromium or molybdenum, of
ordinary photomasks, i.e., about 60 nm to 130 nm. Accordingly, SOG cannot
be etched with sufficiently high accuracy, and it is difficult to obtain
vertical side-walls by etching.
In view of the above-described problems of the background art, it is a
third object of the present invention to provide a halftone phase shift
photomask for which a physical cleaning process used for cleaning ordinary
photomasks can be used as it is and which has vertical side-walls
processed with high accuracy, and also provide a halftone phase shift
photomask blank for producing the halftone phase shift photomask.
SUMMARY OF THE INVENTION
As has been described above, the first object of the present invention is
to provide a halftone phase shift photomask which has a simple structure
and enables the photoengraving process to be shortened, thereby making it
possible to attain a reduction in cost and an improvement in yield, and
which also enables the greater part of a production line for a
conventional chromium photomask to be used as it is, and also provide a
halftone phase shift photomask blank for producing the halftone phase
shift photomask.
The second object of the present invention is to provide a halftone phase
shift photomask which is designed so that:
1 the rise in the transmittance for a long wavelength in the visible
region, which is used for inspection, measurement, etc., is suppressed to
prevent reduction in contrast between the transparent and semitransparent
regions, thereby facilitating inspection and measurement;
2 electric conductivity is imparted to the phase shifter film to thereby
prevent occurrence of charge-up;
3 the control of transmittance is facilitated with the exposure wavelength
phase difference held at 180.degree.;
4 the reflectivities of the obverse and reverse surfaces can be controlled;
and
5 an optimal multilayer structure can be realized for each of at least two
different kinds of exposure light by controlling the thickness of each
layer with the film composition of each layer maintained as it is.
The third object of the present invention is to provide a halftone phase
shift photomask for which a physical cleaning process used for cleaning
ordinary photomasks can be used as it is and which has vertical sections
processed with high accuracy, and also provide a halftone phase shift
photomask blank for producing the halftone phase shift photomask.
In view of the above-described problems of the background art, we conducted
studies in order to develop a halftone phase shift photomask which is
practical, highly accurate and easy to produce. As a result, we have found
that a semitransparent film of a halftone phase shift photomask, which
performs both phase inversion and transmittance control, can be arranged
in the form of a single-layer film or a multilayer film which can be
patterned by a single photoengraving process by adopting a structure
including a film composed mainly of a chromium compound which falls within
a certain composition range. The halftone phase shift photomask and
halftone phase shift photomask blank according to the first aspect of the
present invention have been accomplished on the basis of this finding.
That is, according to the first aspect of the present invention, there is
provided a halftone phase shift photomask blank comprising a transparent
substrate, and a single-layer film provided on the transparent substrate
and composed mainly of a chromium compound which falls within a limited
composition range, or a multilayer film including the chromium compound
film. In addition, there is provided a halftone phase shift photomask
obtained by patterning the semitransparent film of the above-described
blank.
The semitransparent film of a halftone phase shift photomask is required to
have characteristics for inverting the phase of exposure light and
characteristics for controlling the transmittance, as has already been
described above. These characteristics are determined by the complex index
of refraction (refractive index and extinction coefficient) of a substance
constituting the semitransparent film (substances constituting respective
layers, in the case of a multilayer film) and the film thickness. If the
semitransparent film is treated as an absorbing film which is shown in M.
Born, E. Wolf "Principles of Optics", pp.628-632, for example, multiple
interference can be neglected. Accordingly, the phase change .phi. of
perpendicularly transmitted light may be calculated as follows:
##EQU1##
where .phi. is the phase change occurring on light when perpendicularly
transmitted through a photomask having a multilayer (m-2 layers) film
formed on a substrate, x.sup.k,k+1 is the phase change occurring at the
interface between the k-th layer and the (k+1)th layer, u.sub.k and
d.sub.k are the refractive index of a material constituting the k-th layer
and the thickness of the k-th layer, respectively, and .lambda. is the
wavelength of exposure light. It is assumed here that the layer of k=1 is
the substrate, and the layer of k=m is air.
In general, halftone phase shift photomasks are required to have a
transmittance of 1% to 50% for exposure light. If a semitransparent film
formed on a transparent substrate to a thickness obtained by calculation
of the above equation (1) shows a transmittance in the range of 1% to 50%,
the semitransparent film can be used for a single-layer halftone phase
shift photomask as it is. When the transmittance exceeds the upper limit
of the above range (i.e., when the semitransparent film transmits an
excessively large amount of exposure light), a light-shielding layer made
of a metal chromium thin film or the like for controlling the
transmittance is provided in addition to the semitransparent film in a
stack structure, thereby enabling the transmittance to fall within the
above range in the form of the multilayer halftone phase shift photomask
described above.
One of the advantageous features of the film composed mainly of a chromium
compound according to the first aspect of the present invention resides in
that if a film composed mainly of metal chromium, for example, is used as
the above-described light-shielding layer, the semitransparent film can be
patterned by only one photoengraving process, as shown in Examples
(described later), and it is therefore possible to reduce the number of
manufacturing steps required and achieve a reduction in cost and an
improvement in yield. It is a matter of course that transmittance control
and phase inversion can also be effected by a semitransparent film formed
by stacking two or more films composed mainly of chromium compounds having
different compositions, respectively. In such a stack structure, the
compositions of the layers do not necessarily need to change
discontinuously at the interface between each pair of adjacent layers, but
the arrangement may be such that there is substantially no interface and
the composition changes continuously. In such a case, however, the amount
of phase change slightly shifts from that in the simulation of the above
equation (1). Further, each layer does not necessarily need to be formed
of a material which is homogeneous in the direction of the thickness
thereof, but there may be a distribution in the composition, structure,
etc. When a semitransparent film is formed by stacking a plurality of
films including a film composed mainly of a chromium compound and a film
composed mainly of metal chromium, it is possible to control the
transmittance spectrum distribution and improve etching processability by
making the compositions, textural structures and film forming conditions
of these films different from each other.
Examples of the chromium compound used in the halftone phase shift
photomask according to the first aspect of the present invention include
chromium oxide, chromium oxide carbide, chromium oxide nitride, chromium
oxide carbide nitride, and these chromium compounds which contain argon.
It has been found that films which are composed mainly of these compounds
largely vary in optical characteristics according to the chemical
composition thereof. We measured the refractive index and extinction
coefficient of films formed with the composition varied and also the
transmittance at the above film thickness adequate for phase inversion. As
a result, we have found that there is a composition range within which a
film composed mainly of a chromium compound can be effectively used as a
semitransparent film of a single-layer or multilayer halftone phase shift
photomask. Grounds for determining this composition range will be
explained below more specifically.
The film composed mainly of a chromium compound is formed on a transparent
substrate by a conventional thin-film forming method, e.g., vacuum
deposition, sputtering, ion plating, etc. In the case of sputtering, for
example, the film can be formed by reactive sputtering of a metal chromium
target. In the process, a reactive gas, which can supply an element that
will be taken into the film by a reaction taking place on the target
surface or the substrate surface or in the sputter space to form a
chromium compound, is mixed with a conventionally used sputter gas
according to need, thereby forming a chromium compound on the substrate.
It is a matter of course that both the reactive gas and the sputter gas
may be in the form of a mixture of a plurality of gases. This method has a
great advantage in that a sputtering system that is used to produce
ordinary chromium photomask blanks can be used as it is. By changing the
kinds of gas and the mixing ratio, chromium compounds having various
compositions can be formed.
In this type of film forming method, it is conventional practice to use as
a reactive gas one or a plurality of gases selected from among gases which
can serve as an oxygen source gas, e.g., oxygen, carbon dioxide, carbon
monoxide, water vapor, nitrogen oxide, nitrogen suboxide, etc., and one or
a plurality of gases selected from among conventionally used sputter
gases, e.g., argon, nitrogen, neon, helium, xenon, etc., according to
need. FIG. 5 shows the results of an example in which carbon dioxide gas
was used as an oxygen source, while nitrogen gas was used as a sputter
gas, and films were formed on a satisfactorily cleaned silicon wafer with
the carbon dioxide gas/nitrogen gas flow rate ratio varied. Then, the
refractive index and extinction coefficient of each film were measured
with a commercially available spectroellipsometer. Further, a film
thickness required for shifting the phase of exposure light through
180.degree. was calculated with regard to exposure carried out at the
wavelength of the i-line (356 nm) of a super-high pressure mercury lamp,
using a synthetic quartz substrate, and the transmittance of each film
formed on the photomask substrate to the calculated thickness was
measured. The film forming system used in the example was an ordinary
planar DC magnetron sputtering system, and the film formation was carried
out under the conditions that the gas pressure was 3.0 mTorr; the overall
gas flow rate of the carbon dioxide gas and the nitrogen gas was 100 sccm;
and the sputter current density was 0.01 A/cm.sup.2. As will be clear from
FIG. 5, films formed in a region where the proportion of the carbon
dioxide gas flow rate is larger than several % can be favorably used as a
semitransparent film for a halftone phase shift photomask from the
viewpoint of the relation to the transmittance.
FIG. 6 shows the results of measurement of the relationship between the
ratio of chromium atoms to oxygen, carbon and nitrogen atoms and the
carbon dioxide gas/nitrogen gas flow rate ratio by the X-ray photoelectron
spectroscopy (XPS) for the films formed on the silicon wafer with the
carbon dioxide gas/nitrogen gas ratio varied as follows: 0/100, 10/90,
20/80, 70/30, and 100/0. The graph shows the numbers of oxygen, carbon and
nitrogen atoms existing per 100 chromium atoms. The X-ray photoelectron
spectroscopy was carried out under the conditions described later. As will
be clear from FIG. 6, a point of inflection is clearly observed in the
relationship between the carbon dioxide gas/nitrogen gas flow rate ratio
and the film composition as shown by the point a in the graph. That is, in
a region where the proportion of the carbon dioxide gas flow rate is
smaller than the point a, the composition largely depends on the flow rate
ratio. As the carbon dioxide gas flow rate is increased from 0 to 20%, the
number of oxygen atoms per 100 chromium atoms rapidly changes from about
50 to more than 200, while the number of nitrogen atoms rapidly changes
from more than 100 to less than 20. The oxygen and nitrogen atoms are in
complementary relation to each other. Therefore, the total number of
oxygen and nitrogen atoms is always of the order of 200 to 300 per 100
chromium atoms. On the other hand, the number of carbon atoms does not
show a large change on the whole. By comparing FIGS. 5 and 6, the
composition range of chromium, oxygen and nitrogen atoms will become
clear.
Similar film formation was carried out in a carbon dioxide gas/argon gas
system using argon, which is a common sputter gas. Consequently, results
similar to those in the case of the carbon dioxide gas/nitrogen gas system
were obtained. Thus, it has been revealed that with regard to chromium,
oxygen and argon atoms also, the same composition range as the above is an
effective range for forming a semitransparent film of a halftone phase
shift photomask.
Next, to examine the role of carbon atoms, similar film formation was
carried out in an oxygen gas/argon gas system using oxygen gas as an
oxygen source in place of the carbon dioxide gas. At this time also,
results similar to the above were obtained. However, the chemical
resistance of the film was inferior to those formed in the carbon dioxide
gas/nitrogen gas system and the carbon dioxide gas/argon gas system. FIG.
7 shows the reduction in thickness of the films after dipping for 30
minutes in a mixed acid of concentrated sulfuric acid and concentrated
nitric acid in the volume ratio of 10:1, heated to 80.degree. C., which is
commonly used for cleaning photomasks. It will be understood from FIG. 7
that the film formed by using carbon dioxide gas is superior to the film
formed by using oxygen gas in the resistance to the mixed acid used for
cleaning. This nature similarly applies to other chemicals (acids,
alkalis, organic solvents, etc.) used in the mask process.
It should be noted that the film formed in the oxygen gas/argon gas system,
shown in FIG. 7, can be put to practical use, although it is inferior in
the resistance to acids.
FIG. 8 shows the composition of the film formed in the oxygen gas/argon gas
system, analyzed by the X-ray photoelectron spectroscopy. It will be
understood by comparing FIGS. 8 and 6 that the two films are different
from each other in the number of carbon atoms contained therein. Thus, it
may be considered that the above-described chemical resistance is obtained
by the presence of carbon atoms in the film. It should be noted that it is
generally difficult to perform quantitative analysis of carbon atoms, and
that the presence of carbon atoms in the film is confirmed in FIGS. 6 and
8 even when a gas that serves as a carbon source is not positively
introduced. Therefore, it is considered that some background is
superimposed on these results. Accordingly, the present inventors examined
various film forming environments and analytical environments and, as a
result, we have found out that the above-described effect is manifested
when the number of carbon atoms contained in the film is not smaller than
2% of the number of chromium atoms. FIG. 9 shows the profile of the
composition in the direction of the depth obtained by the X-ray
photoelectron spectroscopy. In many cases, many carbon atoms are
distributed in the surface region. A film that contains a relatively large
number of carbon atoms in the surface region has improved resistance to
chemicals used for dipping.
It may be considered from the above results that films composed mainly of a
chromium compound which are usable as a semitransparent film of a halftone
phase shift photomask are restricted to those which fall within the
following composition ranges 1 to 6. Although in the foregoing description
the present invention has been explained by way of one example in which
reactive sputtering using a chromium target is used as a production
process, it should be noted that the described reactive sputtering is
merely an example, and that any type of production process may be
employed. It is considered that any film that falls within any of the
following composition ranges exhibits similar characteristics:
1 a film in which the ratio of the number of chromium atoms to the number
of oxygen atoms is in the range of from 100:100 to 100:300;
2 a film which satisfies the condition 1 and in which the number of carbon
atoms contained is not smaller than 2% of the number of chromium atoms;
3 a film which satisfies the condition 2 and in which a larger number of
carbon atoms are contained in a surface region within the depth of 3 nm
from the film surface than in the other region;
4 a film which satisfies the condition 1 and in which nitrogen atoms are
contained in such a proportion that the total number of nitrogen and
oxygen atoms is not larger than 350 per 100 chromium atoms;
5 a film which satisfies the condition 1 and in which argon atoms are
contained in such a proportion that the total number of argon and oxygen
atoms is not larger than 350 per 100 chromium atoms; and
6 a film which satisfies any of the conditions 1 to 5 and which contains
impurity atoms other than chromium, oxygen, carbon, nitrogen and argon
atoms within the range in which the refractive index for exposure light
that is obtained by ellipsometry will not be changed by 0.1 or more.
In the experimental examples of the halftone phase shift photomask
according to the first aspect of the present invention, the X-ray
photoelectron spectroscopy was carried out as follows:
As an X-ray photoelectron spectroscope, ESCASCOPE, manufactured by VG
SCIENTIFIC, England, was used.
The electron energy analyzer of ESCASCOPE was a 180.degree. concentric
hemispherical analyzer, and an X-ray photoelectron spectrum was measured
by using a 6-channel thoron detector.
The data processor was DEC Micro PDP11/53, and VGS DATA SYSTEM VGS5250
Version January 1992 was used as software to execute quantitative
calculation and other processing.
The analyzer work function of this system was 4.51 eV. The basic
performance of this system was as shown in the table below at 3d5/2 peak
of Ag when measurement was carried out at 400 W using MgK.alpha. (1253.60
eV) as an exciting line X-ray source:
______________________________________
Energy resolving power (eV)
0.95 1.10 1.40 2.20
______________________________________
Sensitivity (kcps/mm.sup.2)
130 260 480 930
______________________________________
The measuring conditions were as follows:
As an X-ray source, the exciting line of AlK.alpha. rays (1486.60 eV) was
used, and measurement was carried out at 500 W.
The angle of incidence of X-rays was 60.degree. from the normal to the
sample. The detector was disposed on the line normal to the sample.
Measurement of the degree of vacuum was carried out by using MILLENIA
SERIES IPGCI. The degree of vacuum was in the range of from
5.times.10.sup.-10 mbar to 1.times.10.sup.-6 mbar. The evacuation system
was composed of an ion pump, StarCell power unit 929-0172 (220 l/s),
manufactured by Varian, and a titanium sublimation pump by VGSPS7
SUBLIMATION PUMP CONTROLLER.
As an analysis region, a region of about 1 mm or less in diameter was
measured.
The XPS spectrum was measured for each binding energy as follows:
Wide scan: 1,000 eV to 0 eV (B. E.)
Cr 2p: 620 eV to 570 eV (B. E.)
O ls: 560 eV to 520 eV (B. E.)
C ls: 320 eV to 270 eV (B. E.)
N ls: 430 eV to 380 eV (B. E.)
All the measuring operations were carried out in the CAE mode. In the wide
scan, the pass energy was 60 eV, with 1 eV step, and the number of times
of scanning was 2. In the other cases, the pass energy was 50 eV, with 0.1
eV step, and the number of times of scanning was 5. In all the cases, the
channel time was 100 ms.
Although these measuring conditions were adopted in the present invention,
it should be noted that these conditions are merely an example, and that
in the case of an ordinary system, measurement may be carried out in any
practically adequate range in which resolving power and sensitive are not
considerably impaired, with the amount of electric charge taken into
consideration. The element composition quantitative calculation procedure
was as follows:
The subtraction of the background was made by using the Shirley type in the
software. Determination of the background was made by giving careful
consideration so that there would be no effect of a satellite of the main
peak and that the most natural peak configuration would be obtained. The
quantitative calculation was made on the basis of Scofield's relative
sensitivity coefficients in the software. That is, the peak area obtained
by measurement was divided by the relative sensitivity coefficient, and
the composition ratio of each element was calculated from the resulting
quotient.
As the composition ratio of each constituent element, the value of the
calculated composition ratio that had become approximately constant
independently of the etching time was employed.
The Scofield's relative sensitivity coefficients are as follows:
Carbon: 1.00
Oxygen: 2.85
Chromium: 7.60
Nitrogen: 1.77
Argon: 3.13
The etching conditions were as follows:
As an ion gun, EX05 differential exhaust type two-stage electrostatic
lens-mounted electron bombardment ion gun was used, and as a controller,
400X gun supply unit was used. The magnification of the physical image
unit was set at 1.
For measurement of the sample current, 626 sample current meter was used.
Etching was carried out in the vacuum range of from 1.times.10.sup.-7 mbar
to 1.times.10.sup.6 mbar and in the sample current range of about -0.5
.mu.A to -1.0 .mu.A.
The filament current was 2.2 A, the emission current was 5 mA to 10 mA, and
the source energy was 3 KV to 5 KV.
As an etching gas, Ar or Ne was used.
The etching time depended on the etch rate of the substrate. Etching and
XPS spectral measurement were alternately carried out until the existence
ratio of each particular element detected was regarded as being
approximately constant.
Charge correction using an electron gun was not carried out.
The above-described measuring conditions and etching conditions in the
present invention are merely an example. In general, measurement of equal
spectrum quality can be made under other conditions as long as sensitivity
and resolving power are not impaired.
As has been described above, the halftone phase shift photomask and
halftone phase shift photomask blank of the present invention have on a
transparent substrate a halftone phase shift layer which includes at least
one layer composed mainly of a chromium compound, wherein the ratio of the
number of chromium atoms to the number of oxygen atoms in the layer
composed mainly of a chromium compound, when measured by X-ray
photoelectron spectroscopy, falls within the range of from 100:100 to
100:300.
In this case, it is preferable that the number of carbon atoms contained in
the layer composed mainly of a chromium compound should be not smaller
than 2% of the number of chromium atoms. In this case, it is preferable
that a larger number of carbon atoms should be contained in a surface
region within the depth of 3 nm from the film surface than in the other
region.
It is also preferable that nitrogen atoms should be contained in such a
proportion that the total number of nitrogen and oxygen atoms is not
larger than 350 per 100 chromium atoms. Alternatively, it is preferable
that argon atoms should be contained in such a proportion that the total
number of argon and oxygen atoms is not larger than 350 per 100 chromium
atoms.
Under the above-described conditions, the layer composed mainly of a
chromium compound may contain impurity atoms other than chromium, oxygen,
carbon, nitrogen and argon atoms within the range in which the refractive
index for exposure light that is obtained by ellipsometry will not be
changed by 0.1 or more.
The layer that constitutes a halftone phase shift layer is preferably
formed on the transparent substrate so that the phase difference .phi.,
which is obtained by the following equation, is n.pi..+-..pi./3 radians (n
is an odd number):
##EQU2##
where .phi. is the phase change occurring on light when perpendicularly
transmitted through a photomask having a multilayer (m-2 layers) film
formed on a substrate, x.sup.k,k+1 is the phase change occurring at the
interface between the k-th layer and the (k+1)th layer, u.sub.k and
d.sub.k are the refractive index of a material constituting the k-th layer
and the thickness of the k-th layer, respectively, and .lambda. is the
wavelength of exposure light. It is assumed here that the layer of k=1 is
the substrate, and the layer of k=m is air.
Further, the layer that constitutes a halftone phase shift layer is
preferably formed on the transparent substrate at such a thickness that
the transmittance for exposure light is in the range of from 1% to 50%.
The present inventors also conducted exhaustive studies in order to develop
a practical and highly accurate halftone phase shift photomask, and as a
result, we have found that the following advantages 1 to 5 are obtained by
forming a semitransparent film of a halftone phase shift photomask in a
multilayer structure, particularly a double- or triple-layer structure
including a chromium nitride layer and a chromium oxide nitride carbide
layer:
1 the transmittance at the long wavelength side can be held down to a
relatively low level;
2 the semitransparent film can be provided with charge-up preventing
properties;
3 the control of transmittance can be facilitated without changing the
phase difference of the exposure wavelength;
4 the reflectivities of the obverse and reverse surfaces can be controlled;
and
5 the film thickness of each layer can be controlled with the film
composition of each layer maintained as it is, thereby realizing an
optimal multilayer structure for each of at least two different kinds of
exposure light.
Thus, the halftone phase shift photomask according to the second aspect of
the present invention has been accomplished on the basis of the above
finding.
That is, according to the second aspect of the present invention, there is
provided a halftone phase shift photomask having on a transparent
substrate a region which is semitransparent to exposure light and a region
which is transparent to the exposure light so that the phase difference
between light passing through the transparent region and light passing
through the semitransparent region is substantially 180.degree.. The
halftone phase shift photomask is characterized in that:
(1) a semitransparent film that constitutes the semitransparent region is
arranged in the form of a double-layer film which includes, in order from
the transparent substrate side, a single-layer film 3 of a compound
selected from among chromium oxide, chromium oxide nitride, chromium oxide
carbide, and chromium oxide nitride carbide, and a single-layer film 4 of
either chromium or chromium nitride, as shown in FIG. 10; or
(2) the semitransparent film is arranged in the form of a double-layer film
which includes, in order from the transparent substrate side, a
single-layer film 5 of either chromium or chromium nitride, and a
single-layer film 6 of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide, as shown in FIG. 11; or
(3) the semitransparent film is arranged in the form of a triple-layer film
which includes, in order from the transparent substrate side, a
single-layer film 7 of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide, a single-layer film 8 of either chromium or chromium nitride, and
a single-layer film 9 of a compound selected from among chromium oxide,
chromium oxide nitride, chromium oxide carbide, and chromium oxide nitride
carbide, as shown in FIG. 12.
The present invention also includes a blank for producing such a halftone
phase shift photomask and methods of producing the halftone phase shift
photomask and the halftone phase shift photomask blank.
In general, the above-described chromium, chromium oxide, chromium nitride,
chromium oxide nitride, chromium oxide carbide and chromium oxide nitride
carbide films are produced by using sputtering. However, vapor deposition,
ion plating, CVD (Chemical Vapor Deposition), etc. may also be used.
The thickness of a multilayer film that constitutes the semitransparent
film should be adjusted so that in the following expression (2), a is in
the range of from 1/3 to 2/3, i.e., 1/3.ltoreq.a.ltoreq.2/3:
a=.SIGMA..sub.i d.sub.i (n.sub.i -1)/.lambda. (2)
where .lambda. is the exposure wavelength, d.sub.i is the thickness of the
i-th layer constituting the semitransparent film, n.sub.i is the
refractive index of the i-th layer for the exposure wavelength, and
.SIGMA..sub.i indicates summation with respect to i. It is the best that a
is in the vicinity of 1/2, as a matter of course. The above matter may be
expressed in another form. That is, the semitransparent film should be
formed on the transparent substrate so that the phase difference .PHI.
between the film and an air layer of the same thickness, which is obtained
by the following equation (3), falls within the range of n.+-./3 radians
(n is an odd number):
##EQU3##
where .PHI. is the phase difference of light perpendicularly passing
through a photomask having a multilayer (m-2 layers) film formed on a
transparent substrate, n.sub.k, d.sub.k are the refractive index and
thickness, respectively, of a material constituting the k-th layer, and
.lambda. is the wavelength of exposure light. It is assumed here that the
layer of k=1 is the transparent substrate, and the layer of k=m (m>3; m is
an integer) is air.
With regard to the transmittance of the semitransparent film, when the
transmittance of the transparent region is assumed to be 100%, the
transmittance of the semitransparent region should be adjusted to be 1% to
50%, preferably 3% to 20%. By doing so, a satisfactorily effective
halftone phase shift photomask can be obtained.
FIG. 14 shows the spectral transmittance of chromium nitride of 14 nm in
thickness in the wavelength range of from 200 nm to 800 nm. As will be
understood from the graph, chromium nitride has flat spectral
characteristics over the wavelength range of from 300 nm to 800 nm.
Accordingly, the transmittance at the long wavelength side can be held
down to a relatively low level by using chromium nitride.
FIG. 13 shows the spectral transmittance in the wavelength range of from
200 nm to 800 nm in the double-layer film of chromium nitride and chromium
oxide according to the second aspect of the present invention. It will be
understood by making comparison with the spectral transmittance in the
single-layer film shown in FIG. 18 that in the double-layer film the
transmittance at the long wavelength side in the visible region is lower
than that in the single-layer film. Accordingly, it is possible to perform
inspection with an ordinary inspection apparatus KLA-219e (manufactured by
KLA) whereby two semiconductor chips are comparatively inspected.
The following is a description of the charge-up preventing effect in
electron beam lithography of an electron beam resist used as a mask for
processing a semitransparent film. If a chromium or chromium nitride film,
which is electrically conductive, is used as any of the upper,
intermediate and lower layers with respect to an ordinary chromium oxide
nitride carbide film, electric charge accumulated on or in the
semitransparent film during electron beam lithography is grounded through
the chromium or chromium nitride film. Thus, charge-up can be prevented.
For the charge-up preventing effect, the above-described structure (1) is
most effective, but the structures (2) and (3) can also produce
satisfactory charge-up preventing effect.
With such a structure, transmittance controllability for the exposure
wavelength also improves. It is difficult to independently control the
refractive index and the extinction coefficient by film forming
conditions. In the case of a single-layer structure, it is necessary in
order to obtain different transmittances of 7%, 10% and 15%, for example,
to set film forming conditions for each transmittance. In the case of a
multilayer structure, on the other hand, the transmittance can be
controlled by the thickness of the chromium or chromium nitride film and
the phase difference can be controlled by the thickness of the chromium
oxide nitride carbide film. Therefore, the transmittance control becomes
easier. The reason for this is as follows. Since the extinction
coefficient of chromium or chromium nitride is large, the transmittance
changes to a considerable extent with a slight change in film thickness,
but the change in the phase difference is small. For a change in the phase
difference, on the other hand, correction can be made with the thickness
of the chromium oxide nitride carbide film.
According to M. Born, E. Wolf "Principles of Optics", pp.628-632, the
transmittance T of an absorbing film is given by
##EQU4##
where n.sub.2 and k.sub.2 are the refractive index and extinction
coefficient, respectively, of the absorbing film for the exposure
wavelength, n.sub.1 and k.sub.1 are the refractive index and extinction
coefficient, respectively, of the entrance-side substance of the absorbing
film for the exposure wavelength, and n.sub.3 and k.sub.3 are the
refractive index and extinction coefficient, respectively, of the
exit-side substance of the absorbing film for the exposure wavelength,
which refractive indices and extinction coefficients satisfy the following
conditions:
##EQU5##
where h is the thickness of the absorbing film, and .lambda. is the
exposure wavelength. Further, it is assumed that normal incidence is
carried out. In the case of a multilayer structure, the overall
transmittance can be calculated from the product of the transmittances of
the constituent layers.
Reflectivity control can be realized when the above-described structure (3)
is adopted. With the structure (3), it is possible to vary the thickness
proportions of the upper and lower chromium oxide nitride carbide films 7
and 9. The spectral reflectivities of chromium and chromium nitride are
only slightly dependent on wavelength and relatively high. The chromium
oxide nitride carbide films 7 and 9, which are formed on the upper and
lower sides of the chromium film 8, serve as anti-reflection films
utilizing multiple interference. Therefore, the interference peak varies
with the film thickness, and hence the reflectivity for the exposure
wavelength can be controlled.
Assuming the structure shown in FIG. 12, the reflectivity R is given by
##EQU6##
where n.sub.2 and k.sub.2 are the refractive index and extinction
coefficient, respectively, of the absorbing film 8 for the exposure
wavelength, n.sub.1 and k.sub.1 are the refractive index and extinction
coefficient, respectively, of the entrance-side substance 7 or 9 for the
exposure wavelength, and n.sub.3 and k.sub.3 are the refractive index and
extinction coefficient, respectively, of the exit-side substance 9 or 7
for the exposure wavelength, which refractive indices and extinction
coefficients satisfy the following conditions:
##EQU7##
where h is the thickness of the absorbing film, and .lambda. is the
exposure wavelength. Further, it is assumed that normal incidence is
carried out.
For processing such a photomask, dry etching by a mixed gas of Cl.sub.2 or
CH.sub.2 Cl.sub.2 and oxygen is generally used. It is also possible to use
a mixed aqueous solution of ammonium ceric nitrate and perchloic acid in
place of the dry etching process.
Even when such a multilayer structure is used, since the upper and lower
layers are formed from films of a similar chromium compound, etching of
the multilayer film can be completed by a single process. Thus, the
multilayer structure invites no increase in the number of processing steps
required.
Incidentally, when the semitransparent film has a structure which includes
at least two layers, it is possible to realize a desired halftone phase
shift photomask for each of two different exposure wavelengths (assumed to
be .lambda..sub.A and .lambda..sub.B) by using materials of the same
composition for the constituent layers and selecting a thickness for each
layer. This will be explained below more specifically.
Let us consider a double-layer structure. In a case where the first layer
(light-shielding layer) and the second layer (shifter layer) are formed by
using a material with which the refractive indices of the first and second
layers for the exposure wavelength .lambda..sub.A are n.sub.1A and
n.sub.2A, respectively, first the thickness h.sub.1A of the first layer is
determined so that the transmittance condition is satisfied (i.e., the
transmittance has a substantial value). Then, the thickness h.sub.2A of
the second layer may be determined so that the phase condition (2) is
satisfied, that is, the following condition is satisfied:
[(n.sub.1A -1)h.sub.1A +(n.sub.2A -1)h.sub.2A ].lambda..sub.A =1/2.+-.1/6(A
)
Next, the procedure for determining the thicknesses h.sub.1B and h.sub.2B
of the first and second layers for the exposure wavelength .lambda..sub.B
will be explained. It is assumed that the same material as the above is
used. In such a case also, the refractive indices of the first and second
layers are n.sub.1B and n.sub.2B, which are different in values from
n.sub.1A and n.sub.2A. First, h.sub.1B is determined so that the
transmittance condition is satisfied. Then, h.sub.2B may be determined so
that the phase condition (2) is satisfied, that is, the following
condition is satisfied:
[(n.sub.1B -1)h.sub.1B +(n.sub.2B -1)h.sub.2B ].lambda..sub.B =1/2.+-.1/6(B
)
On the other hand, in the case of a single-layer structure, since h.sub.2A
=h.sub.2B =0, even when it is possible to determine n.sub.1A and h.sub.1A
which satisfy the transmittance condition and the phase condition, that
is, Equation (A), h.sub.2A which satisfies the phase condition, that is,
Equation (B), is uniquely determined (because n.sub.2A is uniquely
determined, in the case of the same material). Accordingly, it becomes
impossible to satisfy the transmittance condition.
The present inventors also conducted exhaustive studies in order to develop
a halftone phase shift photomask for which a physical cleaning process,
e.g., brush cleaning, high-pressure water cleaning, ultrasonic cleaning,
etc., which is used for cleaning ordinary photomasks, can be used as it is
to effect satisfactory cleaning and which has vertical sections processed
with high accuracy, and also a halftone phase shift photomask blank for
producing the halftone phase shift photomask. As a result, we have come to
accomplish the halftone phase shift photomask and halftone phase shift
photomask blank according to the third aspect of the present invention.
That is, according to the third aspect of the present invention, there is
provided a halftone phase shift photomask having on a transparent
substrate a transparent region and a semitransparent region which is
defined by a semitransparent layer of a chromium compound formed by
physical vapor deposition, wherein when the transmittance of the
transparent region for the wavelength of exposure light applied during
transfer is assumed to be 100%, the transmittance of the semitransparent
region falls within the range of from 3% to 35%, and the semitransparent
region shifts the phase of the exposure wavelength through substantially
180.degree. with respect to the transparent region.
In this case, it is preferable that the chromium compound should consist
essentially of chromium and oxygen, or chromium, oxygen and nitrogen, or
chromium, oxygen and carbon, or chromium, oxygen, nitrogen and carbon, and
that the semitransparent layer should be formed from a single-layer film
or a multilayer film including two or more layers.
In addition, according to the third aspect of the present invention, there
is provided a halftone phase shift photomask blank having on a transparent
substrate a semitransparent layer of a chromium compound which is formed
by physical vapor deposition, wherein when the transmittance of the
transparent substrate for the wavelength of exposure light applied during
transfer is assumed to be 100%, the transmittance of the semitransparent
layer falls within the range of from 3% to 35%, and the amount of phase
shift of the exposure wavelength by the semitransparent layer is
substantially 180.degree..
In this case, it is preferable that the chromium compound should consist
essentially of chromium and oxygen, or chromium, oxygen and nitrogen, or
chromium, oxygen and carbon, or chromium, oxygen, nitrogen and carbon, and
that the semitransparent layer should be formed from a single-layer film
or a multilayer film including two or more layers.
Examples of physical vapor deposition for forming a chromium compound as a
semitransparent layer include vacuum film forming methods such as
sputtering, ion plating, vapor deposition, etc. In the present state of
the art, however, sputtering is particularly preferable. When the
transmittance of the transparent region or the transparent substrate is
assumed to be 100%, the transmittance of the semitransparent layer should
preferably fall within the range of from 3% to 35%, more preferably in the
range of from 5% to 20%.
The chromium compound consists essentially of chromium and oxygen, or
chromium, oxygen and nitrogen, or chromium, oxygen and carbon, or
chromium, oxygen, nitrogen and carbon. The chromium compound may further
contain a transition metal other than chromium, e.g., titanium, tungsten,
tantalum, molybdenum, etc.
By arranging the semitransparent layer in the form of a multilayer film
including two or more layers, it becomes possible to control etching
characteristics during etching process on the basis of the difference in
textural structure and component ratio between the constituent layers.
Accordingly, the etching characteristics can be adjusted so that it is
possible to obtain an even more vertical etched section than in the case
of a single-layer film under the same continuous etching conditions.
Examples of preferable etching methods for the semitransparent layer are an
ordinary wet etching process that uses a mixed aqueous solution of
ammonium ceric nitrate and perchloic acid, and a dry etching process that
uses a mixed gas prepared by adding oxygen to Cl.sub.2, CCl.sub.4,
CHCl.sub.3, CH.sub.2 Cl.sub.2, etc.
The thickness of a multilayer film that constitutes the semitransparent
film should be adjusted so that a=1/2 in the following expression:
a=.SIGMA..sub.i t.sub.i (n.sub.i -1/.lambda.
where .lambda. is the wavelength of exposure light, t.sub.i is the
thickness of each layer, n.sub.i is the refractive index of each layer for
the exposure wavelength, and .SIGMA..sub.i indicates summation with
respect to i.
By doing so, the amount of phase shift becomes substantially 180.degree..
It should be noted that the effect of the phase shift layer is recognized
when a is in the range of from 1/3 to 2/3, i.e., 1/3.ltoreq.a.ltoreq.2/3.
However, it is the best that a is in the vicinity of 1/2, as a matter of
course.
In the halftone phase shift photomask and halftone phase shift photomask
blank according to the first aspect of the present invention, the
structure is simple, and the photoengraving process can be shortened.
Accordingly, it is possible not only to achieve a reduction in cost and an
improvement in yield but also to use the greater part of a production line
for conventional chromium photomasks as it is. Therefore, the halftone
phase shift photomask and halftone phase shift photomask blank can be put
to practical use extremely easily.
In the halftone phase shift photomask and halftone phase shift photomask
blank according to the second aspect of the present invention, the
semitransparent film of the halftone phase shift photomask is arranged in
a multilayer structure which includes one layer of a compound selected
from chromium oxide, chromium oxide nitride, chromium oxide carbide, and
chromium oxide nitride carbide, and one layer of chromium or chromium
nitride. Accordingly, even when the transmittance of the chromium oxide,
chromium oxide nitride, chromium oxide carbide or chromium oxide nitride
carbide layer is relatively high at the long wavelength side, which is
used for inspection and measurement of photomasks, it can take charge of
the phase shifting function (that is, serve as a phase shift layer). On
the other hand, the function of suppressing the rise of the transmittance
at the long wavelength side can be assigned to the chromium or chromium
nitride layer (that is, it serves as a transmittance control layer).
Therefore, it is possible to control not only the transmittance for the
wavelength of exposure light but also the transmittance at the long
wavelength side as desired. Accordingly, the transmittance for the e-line,
which is used for ordinary inspection and size measurement, can also be
held down 30% or less. Thus, ordinary inspection and size measurement can
be performed without problem.
In addition, since the multilayer structure includes an electrically
conductive film, it is possible to prevent charge-up during electron beam
exposure.
Further, since the semitransparent film has a multilayer structure formed
from chromium compounds having different optical constants, the
transmittance and reflectivity can be controlled by combination of film
thicknesses.
Further, an optimal multilayer structure can be realized for each of at
least two different kinds of exposure light by controlling the thickness
of each layer with the film composition of each layer maintained as it is.
According to the third aspect of the present invention, the semitransparent
layer of the halftone phase shift photomask is made of a chromium compound
which is formed by physical vapor deposition. Accordingly, a physical
cleaning process, e.g., brush cleaning, high-pressure water cleaning,
ultrasonic cleaning, etc., which has heretofore been used for cleaning
photomasks, can be used therefor as it is. It is therefore possible to
produce a clean halftone phase shift photomask having quality equal or
close to that of ordinary photomasks and also possible to obtain a clean
halftone phase shift photomask blank. In addition, since the
semitransparent layer of the halftone phase shift photomask has a
thickness not larger than a half of the thickness of the conventional
semitransparent layer formed by using coating glass, it is possible to
produce a highly accurate halftone phase shift photomask having even more
vertical processed sections.
Still other objects and advantages of the invention will in part be obvious
and will in part be apparent from the specification.
The invention accordingly comprises the features of construction,
combinations of elements, and arrangement of parts which will be
exemplified in the construction hereinafter set forth, and the scope of
the invention will be indicated in the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view showing the sequence of steps for processing a
single-layer halftone phase shift photomask in Example 1 according to the
first aspect of the present invention.
FIG. 2 is a sectional view showing the sequence of steps for processing a
multilayer halftone phase shift photomask in Example 2.
FIG. 3 shows the principle of halftone phase shift lithography.
FIG. 4 shows the principle of a conventional method.
FIG. 5 is a graph showing the transmittance for the i-line of a chromium
compound film formed with the carbon dioxide gas/nitrogen gas flow rate
ratio varied, measured at a film thickness adequate for phase inversion.
FIG. 6 is a graph showing the results of composition analysis of the film
shown in FIG. 5 by X-ray photoelectron spectroscopy.
FIG. 7 is a graph showing the way in which the reduction in the film
thickness caused by dipping in chemicals differs according to the kind of
oxygen source gas used.
FIG. 8 is a graph showing the results of composition analysis by X-ray
photoelectron spectroscopy of a chromium compound film formed with the
oxygen gas/nitrogen gas flow rate ratio varied.
FIG. 9 is a graph showing the depthwise profile of the composition obtained
by X-ray photoelectron spectroscopy.
FIG. 10 is a sectional view schematically showing one form of the halftone
phase shift photomask according to the second aspect of the present
invention.
FIG. 11 is a sectional view schematically showing another form of the
halftone phase shift photomask.
FIG. 12 is a sectional view schematically showing still another form of the
halftone phase shift photomask.
FIG. 13 is a graph showing the spectral transmittance of a double-layer
film including chromium nitride and chromium oxide layers according to the
second aspect of the present invention.
FIG. 14 is a graph showing the spectral transmittance of chromium nitride.
FIG. 15 is a graph showing the changes of transmittance for the i-line
relative to the thickness of the light-shielding layer in the case of the
layer configuration shown in FIG. 12.
FIG. 16 is a graph showing the changes of reflectivity for the i-line
relative to the thickness of the entrance-side layer in the case of the
layer configuration shown in FIG. 12.
FIG. 17 is a sectional view showing the structure of a conventional
halftone phase shift photomask.
FIG. 18 is a graph showing the spectral transmittance of chromium oxide.
FIG. 19 is a sectional view schematically showing a halftone phase shift
photomask in one example according to the third aspect of the present
invention.
FIG. 20 is a graph showing the spectral transmittance in the
semitransparent region of the halftone phase shift photomask in the
example shown in FIG. 19.
FIG. 21 is a sectional view showing the process sequence for producing the
halftone phase shift photomask in the example shown in FIG. 19.
FIG. 22 is a sectional view of a halftone phase shift photomask blank in
another example according to the third aspect of the present invention.
FIG. 23 is a sectional view of one example of conventional halftone phase
shift photomasks.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following is a description of examples of the halftone phase shift
photomask, halftone phase shift photomask blank and producing methods
therefor according to the present invention. In particular, Examples 1 and
2 correspond to the techniques according to the first aspect of the
present invention, Examples 3 to 7 to those according to the second aspect
of the present invention, and Examples 8 and 9 to those according to the
third aspect of the present invention.
EXAMPLE 1
One example of a single-layer halftone phase shift photomask will be
described below with reference to FIG. 1, which shows the process sequence
for producing it. As shown in FIG. 1(a), on a mirror-polished silicon
wafer 801, a chromium compound film 802 was formed to a thickness of about
50 nm by sputtering under the following conditions, thereby obtaining a
sample 807 for ellipsometry:
Film forming system: planar DC magnetron sputtering system
Target: metal chromium
Gas and flow rate: carbon dioxide gas 70 sccm+nitrogen gas 30 sccm
Sputter pressure: 3 mTorr
Sputter current: 6 A
Next, the sample 807 was measured for the refractive index u and extinction
coefficient k at the i-line wavelength (365 nm) of a mercury lamp with a
commercially available spectroellipsometer (ES-4G, manufactured by Sopra).
The results were u=2.580 and k=0.374. With the sample 807 treated as a
metal film shown in M. Born, E. Wolf "Principles of Optics", pp.628-632,
mentioned above, a film thickness required for 180.degree. shifting the
phase of transmitted light of wavelength 365 nm by the film when formed on
a high-purity synthetic quartz substrate used as a photomask substrate was
calculated. The required film thickness was determined to be 118 nm.
Accordingly, as shown in FIG. 1(b), a chromium compound film 804 was formed
to a thickness of about 120 nm on an optically polished, satisfactorily
cleaned high-purity synthetic quartz substrate 803 under the
above-described film forming conditions, thereby obtaining a blank 808 for
a single-layer halftone phase shift photomask according to the present
invention wherein the transmittance for light of wavelength 365 m was
about 15%.
It should be noted that the chromium compound was chromium oxide, nitride
carbide.
Next, as shown in FIG. 1(c), the surface of the blank 808 was provided with
a desired resist pattern 805 of a material containing an organic substance
as a principal component by conventional electron beam lithography or
photolithography. Next, as shown in FIG. 1(d), the semitransparent film
804 exposed through the openings in the resist pattern 805 was subjected
to a high-frequency plasma using a mixed gas of CH.sub.2 Cl.sub.2 :O.sub.2
=1:2.5 under the pressure of 0.3 Torr to effect selective dry etching,
thereby obtaining a desired semitransparent film pattern 806. Finally, the
remaining resist was removed by a conventional method, thus obtaining a
single-layer halftone phase shift photomask 809 of the present invention,
as shown in FIG. 1(e).
The single-layer halftone phase shift photomask was practicable in terms of
all requirements, i.e., the dimensional accuracy of the etched portions,
cross-sectional configuration, film thickness distribution, transmittance
distribution, adhesion of the film to the substrate, etc.
EXAMPLE 2
One example of a multilayer halftone phase shift photomask will be
described below with reference to FIG. 2, which shows the process sequence
for producing it. In the same way as in Example 1, a sample for
ellipsometry was prepared and measured for the refractive index and
extinction coefficient, and a film thickness required for 180.degree.
shifting the phase of light of wavelength 365 nm was calculated on the
basis of the measured refractive index and extinction coefficient.
Thereafter, as shown in FIG. 2(a), a chromium compound 902 was actually
formed to the calculated film thickness on a high-purity synthetic quartz
substrate 901 under the following conditions. As a result, the
transmittance for light of wavelength 365 nm was about 20%:
Film forming system: planar DC magnetron sputtering system
Target: metal chromium
Gas and flow rate: carbon dioxide gas 20 sccm+nitrogen gas 80 sccm
Sputter pressure: 3 mTorr
Sputter current: 6 A
Next, a metal chromium film 903 was formed to a thickness of about 10 nm on
the chromium compound 902 under the following conditions, thus obtaining a
blank 906 for a multilayer halftone phase shift photomask according to the
present invention wherein the transmittance for light of wavelength 365 nm
was about 12%:
Film forming system: planar DC magnetron sputtering system
Target: metal chromium
Gas and flow rate: argon gas 100 sccm
Sputter pressure: 3 mTorr
Sputter current: 6 A
Next, as shown in FIG. 2(b), the surface of the blank 906 was provided with
a desired resist pattern 904 of a material containing an organic substance
as a principal component by a conventional electron beam lithography or
photolithography. Next, as shown in FIG. 2(c), the semitransparent film
exposed through the openings in the resist pattern 904 was subjected to a
high-frequency plasma using a mixed gas of CH.sub.2 Cl.sub.2 :O.sub.2
=1:2.5 under the pressure of 0.3 Torr to effect selective dry etching,
thereby obtaining a desired semitransparent film pattern 909 composed of a
chromium compound pattern 905 and a metal chromium pattern 908. Finally,
the remaining resist was removed by a conventional method, thus obtaining
a multilayer halftone phase shift photomask 907 of the present invention,
as shown in FIG. 2(d).
Since the layer 902 formed mainly of a chromium compound and the metal
chromium film 903 have the same matrix comprised of chromium atoms, their
etching characteristics are almost the same. Therefore, the patterning
characteristics of the multilayer halftone phase shift photomask 907 are
approximately the same as those of a single-layer halftone phase shift
photomask such as that shown in Example 1.
This double-layer halftone phase shift photomask was also practicable in
terms of all requirements, i.e., the dimensional accuracy, cross-sectional
configuration, film thickness distribution, transmittance distribution,
adhesion of the film to the substrate, etc.
EXAMPLE 3
First, a CrO.sub.x N.sub.y C.sub.z film was formed to a thickness of 125 nm
on an optically polished quartz substrate. The transmittance of this film
was 13% for the i-line (365 nm) and 45% for the e-line (546 nm). Next, a
CrN film was formed to a thickness of 9 nm on the CrO.sub.x N.sub.y
C.sub.z film. The transmittance of the CrN film was 60% for the i-line and
59% for the e-line. The overall transmittance of the double-layer film was
7.8% for the i-line and 26.6% for the e-line. The refractive index of the
CrN film for the i-line was 1.9, while the refractive index of the
CrO.sub.x N.sub.y C.sub.z film for the i-line was 2.4. In general,
sputtering is used for forming these films.
Next, i-line resist NPR-895i (manufactured by Nagase Sangyo k. k.) was
spin-coated on the blank thus obtained and then subjected to pre-bake,
thereby forming a resist layer having a thickness of 0.1 .mu.m to 2.0
.mu.m. As a substrate, quartz and high-purity quartz substrates are
preferably used. However, it is also possible to use low-expansion glass,
MgF.sub.2, CaF.sub.2, etc. Heating of the resist layer is usually carried
out for from 5 minutes to 60 minutes in the temperature range of from
80.degree. C. to 150.degree. C., although it depends on the kind of resist
used.
Next, a desired pattern is formed by exposure using a laser aligner, e.g.,
CORE-2564 (manufactured by Etec Systems Inc.). Then, the resist is
developed with a developer containing TMAH (tetramethylammonium hydride)
as a principal component, and thereafter rinsed in water.
Next, according to need, heating and descum treatments are carried out to
remove resist scum from the resist pattern. Thereafter, portions of the
semitransparent film exposed through the openings in the resist pattern,
which are to be processed, are subjected to dry etching by an etching
plasma using CH.sub.2 Cl.sub.2 +O.sub.2 gas, thereby forming a
light-shielding pattern. It will be apparent that the formation of the
light-shielding pattern may be effected by wet etching in place of the dry
etching using etching gas plasma.
After the etching process, the remaining resist is removed by an oxygen
plasma, thus completing a halftone phase shift photomask.
The transmittance of this mask was 10% for the i-line (365 nm) and 29% for
the e-line. It was possible to perform inspection without problem by using
a comparative inspection apparatus KLA-219HRL-PS (manufactured by KLA). It
was also possible to carry out size measurement without problem using a
transmission size measuring apparatus MPA MPA-3 (manufactured by Nikon
Corporation).
Next, the transfer characteristic improving effect produced by using the
phase shift photomask of the present invention will be explained below.
With regard to the transfer of a hole pattern of 0.4 .mu.m to a 1 .mu.m
thick positive novolac resist on a wafer by using a 5:1 reduction
projection aligner (N. A.=0.5) using the i-line (wavelength: 365 nm), for
example, we examined the depth of focus (the out-of-focus allowance for
obtaining an opening of 0.4 .mu.m) for a transfer process using an
ordinary photomask and for a transfer process using the phase shift
photomask of the present invention. As a result, it was revealed that with
the ordinary photomask, a depth of focus of only 0.6 .mu.pm was obtained
on the wafer, whereas, with the phase shift photomask of the present
invention wherein the phase difference for the i-line was 180.degree. and
the transmittance for the i-line was 10%, it was possible to obtain a
depth of focus of 1.2 .mu.m on the wafer. Thus, it is possible to form a
hole of 0.4 .mu.m on a wafer for an actual device having a step of about 1
.mu.m with a reduction projection aligner using the i-line.
With a reduction projection aligner using the g-line (wavelength: 436 nm),
it was also possible to improve the depth of focus by using the phase
shift photomask of the present invention having a phase difference and
transmittance optimized for the g-line in the same way as in the case of
the i-line.
With the exposure method using the phase shift photomask of the present
invention, the depth of focus is improved, and it becomes possible to
prevent occurrence of an exposure failure. Therefore, it is possible to
improve the yield in the process of producing semiconductor devices. The
exposure method can be effectively used in the process of producing
semiconductor devices such as 4M-, 16M-, 64M- and 256M-bit DRAMs, SRAMs,
flash memories, ASICs, microcomputers and GaAs devices. It can also
satisfactorily be used in the process of producing unit semiconductor
devices and liquid crystal display devices.
EXAMPLE 4
The triple-layer structure shown in FIG. 12 is used as a layer
configuration of a semitransparent film.
The exposure wavelength is assumed to be 365 nm. Assuming that the
substrate 1 is a quartz substrate, the refractive index thereof is 1.475.
Assuming that the films 7 and 9 are made of chromium oxide nitride carbide
and the film 8 is made of chromium nitride, the refractive indices and
extinction coefficients are as follows: in order from the substrate side,
n.sub.0 : 1.475; n.sub.1 : 2.46; k.sub.1 : 0.29; n.sub.2 : 1.94; k.sub.2 :
3.15; n.sub.3 : 2.46; k.sub.3 : 0.29; and n.sub.4 : 1. It is further
assumed that the thicknesses of the films 7, 8 and 9 are h.sub.1, h.sub.2
and h.sub.3, respectively. Assuming that h.sub.1 =70 nm, h.sub.2 =5 nm,
and h.sub.3 =54.9 nm, the phase difference .PHI. between the region where
the triple-layer film is present and a region where no film is present is
171.degree. when the interface phase difference is taken into
consideration. In this case, there is no change in the phase difference
even if the thickness of the film 8 is changed by .DELTA.h.sub.2, while
the thickness of the film 9 is changed by .DELTA.h.sub.3 (it should be
noted that .DELTA.h.sub.2 and .DELTA.h.sub.3 both indicate an increase in
the film thickness when positive and a reduction in the film thickness
when negative) so that the relationship of (n.sub.2 -1).DELTA.h.sub.2
+(n.sub.3 -1).DELTA.h.sub.3 =0 is satisfied. FIG. 15 is a graph showing
the changes of transmittance when h.sub.2 and h.sub.3 were changed so as
to satisfy the above relationship. The transmittance was calculated from
Equation (2) and the optical constant of each of the above-described
films.
It will be understood from FIG. 15 that a triple-layer structure such as
that shown in FIG. 12 enables the transmittance to be controlled with the
phase difference maintained simply by varying the thicknesses of the two
films constituting two layers without changing the film quality. Although
the films 7 and 9 are formed by using chromium oxide nitride carbide,
these films may be formed of other materials. That is, the film 7 may be
formed by using one material selected from among chromium oxide, chromium
oxide nitride, chromium oxide carbide, and chromium oxide nitride carbide.
The film 8 may be formed by using either chromium or chromium nitride. The
film 9 may be formed by using one material selected from among chromium
oxide, chromium oxide nitride, chromium oxide carbide, and chromium oxide
nitride carbide. The films 7 and 9 do not necessarily need to be formed of
the same material.
Although in this example the semitransparent film has been explained by
using a triple-layer structure as shown in FIG. 12, it should be noted
that similar control can be effected by using a double-layer structure
such as that shown in FIG. 11.
EXAMPLE 5
The triple-layer structure shown in FIG. 12 is used as a layer
configuration of a semitransparent film.
The exposure wavelength is assumed to be 365 nm. Assuming that the
substrate 1 is a quartz substrate, the refractive index thereof is 1.475.
Assuming that the films 7 and 9 are made of chromium oxide nitride carbide
and the film 8 is made of chromium nitride, the refractive indices and
extinction coefficients are as follows: in order from the substrate side,
n.sub.0 : 1.475; n.sub.1 : 2.46; k.sub.1 : 0.29; n.sub.2 : 1.94; k.sub.2 :
3.15; n.sub.3 : 2.46; k.sub.3 : 0.29; and n.sub.4 : 1. It is further
assumed that the thicknesses of the films 7, 8 and 9 are h.sub.1, h.sub.2
and h.sub.3, respectively. Assuming that h.sub.1 =8 nm, h.sub.2 =10 nm,
and h.sub.3 =120 nm, the phase difference .PHI. between the region where
the triple-layer film is present and a region where no film is present is
181.degree. when the interface phase difference is taken into
consideration. In this case, there is no change in the phase difference
even if the thicknesses h.sub.1 and h.sub.3 of the films 7 and 9 are
changed so that the relationship of h.sub.1 +h.sub.3 =128 nm is satisfied.
FIG. 16 is a graph showing the changes of the reflectivities of the
obverse and reverse surfaces of the mask when h.sub.1 and h.sub.3 were
changed so as to satisfy the above relationship. The reflectivities were
calculated from Equation (3) and the optical constant of each of the
above-described films.
It will be understood from FIG. 16 that a triple-layer structure such as
that shown in FIG. 12 enables the reflectivities of the obverse and
reverse surfaces of the mask to be controlled by varying the thicknesses
of the two films. Although the films 7 and 9 are formed by using chromium
oxide nitride carbide, these films may be formed of other materials. That
is, the film 7 may be formed by using one material selected from among
chromium oxide, chromium oxide nitride, chromium oxide carbide, and
chromium oxide nitride carbide. The film 8 may be formed by using either
chromium or chromium nitride. The film 9 may be formed by using one
material selected from among chromium oxide, chromium oxide nitride,
chromium oxide carbide, and chromium oxide nitride carbide. The films 7
and 9 do not necessarily need to be formed of the same material.
EXAMPLE 6
On an optically polished quartz substrate, a chromium oxide nitride carbide
film having a refractive index of 2.50 for the i-line and an extinction
coefficient of 0.27 for the i-line is formed to a thickness of 116 nm by
carrying out reactive sputtering using a chromium target and a mixed gas
of nitrogen and carbon dioxide gas as a sputter gas while controlling the
gas flow rate ratio. Next, a chromium nitride film having a refractive
index of 1.93 for the i-line and an extinction coefficient of 3.15 for the
i-line is formed to a thickness of 10 nm by carrying out reactive
sputtering using argon and nitrogen while controlling the gas flow rate
ratio. The sheet resistance of the film is 26 ohm/sq. The transmittance
for the i-line is 7.0%.
Next, an electron beam resist EBR 900 (manufactured by Toray Industries,
Inc.) is spin-coated on the blank and subjected to pre-bake at 110.degree.
C., thereby forming a resist layer of 500 nm in thickness.
Next, a predetermined pattern is formed by exposure using an electron beam
exposure system MEBESIV (manufactured by Etec Systems Inc.). Since the
uppermost layer is formed of chromium nitride, which is electrically
conductive, charge-up is prevented during the exposure. Therefore, no
displacement of the resist pattern occurs.
Next, the resist layer is developed with a developer containing TMAH
(tetramethylammonium hydride) as a principal component, and thereafter
rinsed in water, thereby forming a resist pattern.
Next, according to need, heating and descum treatments are carried out to
remove resist scum from the resist pattern. Thereafter, portions of the
semitransparent film exposed through the openings in the resist pattern,
which are to be processed, are subjected to dry etching by an etching
plasma using CH.sub.2 Cl.sub.2 +O.sub.2 gas, thereby forming a
semitransparent pattern.
After the above etching process, the remaining resist is removed by an
oxygen plasma, followed by cleaning, inspection and correction. Thus, a
favorable halftone phase shift photomask is completed.
Although in this example the halftone phase shift photomask has been
described by using a double-layer structure as shown in FIG. 10, similar
control can be effected by using a triple-layer structure such as that
shown in FIG. 12.
EXAMPLE 7
The double-layer structure shown in FIG. 11 was used. The film 5 was formed
of chromium nitride, while the film 6 was formed of chromium oxide nitride
carbide. Two exposure wavelengths were used: .lambda..sub.i =365 nm (the
i-line), and .lambda..sub.g =436 nm (the g-line). The refractive indices
and extinction coefficients of the films 5 and 6 for the i-line were as
follows: n.sub.1i =1.94, k.sub.1i 3.15 n.sub.2i 2.47 and k.sub.2i =0.29.
On the other hand, the refractive indices and extinction coefficients for
the g-line of films 5 and 6, which had the same compositions as those of
the above films 5 and 6, were as follows: n.sub.1g =2.36, k.sub.1g =3.51,
n.sub.2g =2.38, and k.sub.2g =0.14.
Accordingly, as a layer configuration for the i-line, films 5 and 6 were
formed so that their respective thicknesses were h.sub.1i =10 nm and
h.sub.2i =125 nm, and as a layer configuration for the g-line, films 5 and
6 were formed so that their respective thicknesses were h.sub.1g =15 nm
and h.sub.2g =160 nm. Thus, it was possible to produce blanks having a
phase difference of about 180.degree. for the i- and g-lines and a
transmittance of 6% to 8% for the i- and g-lines.
Let us evaluate the phase difference on the basis of the value of a in
Equation (2) for the sake of simplification, although the phase difference
should be evaluated on the basis of Equation (1) using the above numerical
values. For the i-line, a=0.53; for the g-line, a=0.55. Thus, a is close
to 1/2, and it is therefore confirmed that the selection of film
thicknesses is rational.
The double-layer structure in this example has the following advantages:
(1) Blanks for the i-line and the g-line can be produced simply by varying
the period of time for film formation without changing the film forming
conditions. Accordingly, production stability improves, and the process
control is facilitated.
(2) Since there are only several materials to be subjected to
characteristic evaluation, the quality assurance is facilitated.
EXAMPLE 8
FIG. 19 is a sectional view schematically showing a halftone phase shift
photomask in one example of the present invention. On a transparent
substrate 301 of optically polished quartz glass, a first semitransparent
film 302 is formed to a thickness of 70 nm by PVD using a chromium
compound composed of chromium, oxygen and nitrogen, and a second
semitransparent film 303 is stacked thereon to a thickness of 65 nm by a
similar method using a chromium compound composed of chromium, oxygen,
nitrogen and carbon. The first and second transparent films 302 and 303
are patterned to constitute a semitransparent layer pattern 304. It should
be noted that the refractive indices of the first and second
semitransparent films 302 and 303 for 356 nm were 2.3 and 2.4,
respectively. It was possible to adjust the refractive index of each film
by about 20% by controlling the film forming conditions.
FIG. 20 is a graph showing one example of the spectral transmittance in the
wavelength range of from 200 nm to 800 nm in the region of the
semitransparent layer 304 of the halftone phase shift photomask in this
example. The transmittance at 365 nm has been adjusted to 11.5%, which
falls within the range of from 3% to 35%. It was possible to adjust the
transmittance of each layer by controlling the film forming conditions and
the film thickness.
When an etched portion of the semitransparent layer 304, composed of two
layers, on the transparent substrate 301 formed in this example was
obliquely observed from above it with a scanning electron microscope
(SEM), it was revealed that the end face of the semitransparent layer was
divided into two portions and the end portions had been adjusted so as to
align perpendicularly to the substrate surface.
FIGS. 21(a) to 21(c) are sectional views showing the process sequence for
producing the halftone phase shift photomask in this example. First, as
shown in FIG. 21(a), on a transparent substrate 301 of optically polished
quartz glass, a first semitransparent film 302 was formed to a thickness
of 70 nm by sputtering using a chromium target and a mixed gas of argon,
oxygen and nitrogen, and a second semitransparent film 303 was stacked
thereon to a thickness of 65 nm by sputtering using a mixed gas of argon,
oxygen, nitrogen, and carbon dioxide, thereby completing a halftone phase
shift photomask blank having a semitransparent layer 304 formed from the
double-layer film.
The refractive indices of the first and second semitransparent films 302
and 303 for the wavelength of 365 nm were 2.3 and 2.4, respectively. The
transmittance in the wavelength range of from 200 nm to 800 nm was similar
to that shown in FIG. 20.
Next, as shown in FIG. 21(b), a desired resist pattern 306 of a material
containing an organic substance as a principal component was formed on the
semitransparent layer 304 by using the conventional electron beam
lithography or photolithography.
Next, as shown in FIG. 21(c), the semitransparent layer 304 exposed through
the openings in the resist pattern 306 was continuously etched by dry
etching using a mixed gas prepared by adding oxygen to Cl.sub.2,
CCl.sub.4, CHCl.sub.3, CH.sub.2 Cl.sub.2, etc., thereby forming a desired
opening pattern 305. Finally, the remaining resist 306 was removed by
plasma ashing or wet releasing process, thus completing one example of the
halftone phase shift photomask according to the present invention as shown
in FIG. 19.
EXAMPLE 9
FIG. 22 is a sectional view of another example of the halftone phase shift
photomask blank. In this example, on a transparent substrate 301 of
optically polished quartz glass, a first semitransparent film 307 of a
chromium compound composed of chromium, oxygen, nitrogen and carbon was
formed to a thickness of 70 nm to 90 nm by sputtering similar to that in
Example 8, and subjected to vacuum heat treatment for about 10 minutes to
30 minutes at a temperature of 200.degree. C. to 400.degree. C. to improve
the film texture and the film surface. Thereafter, a second
semitransparent film 308 of a chromium compound was formed to a thickness
of 40 nm to 70 nm on the first semitransparent film 307 by a method
similar to that used to form the first semitransparent film 307, thereby
forming a semitransparent film 309. Thus, a halftone phase shift photomask
blank of Example 9 of the present invention was completed.
In this example, since the second semitransparent film 308 is formed after
the surface of the first semitransparent film 307 has been modified, the
columnar texture is finer than in the case of a semitransparent layer
formed from a single layer at a time. For this reason, the halftone phase
shift photomask blank of Example 9 having the semitransparent layer 309
formed from the double-layer film was superior in the etching accuracy.
Although in Example 9 a chromium compound composed of chromium, oxygen,
nitrogen and carbon was used, it is also possible to use a chromium
compound composed of chromium and oxygen, or chromium, oxygen and
nitrogen, or chromium, oxygen and carbon, as a matter of course.
As will be clear from the foregoing description, the halftone phase shift
photomask and halftone phase shift photomask blank according to the first
aspect of the present invention are simple in structure and enable the
photoengraving process to be shortened. Therefore, it is possible not only
to achieve a reduction in cost and an improvement in yield but also to use
the greater part of a production line for conventional chromium masks as
it is. Accordingly, the halftone phase shift photomask and halftone phase
shift photomask blank can be put to practical use extremely easily.
The halftone phase shift photomask and halftone phase shift photomask blank
according to the second aspect of the present invention can be produced by
using the same production process, the same inspection process and the
same correction process as those for ordinary photomasks. Accordingly,
halftone phase shift photomasks, which have heretofore been suffering from
a low yield and a high cost due to an increase in the number of
manufacturing steps and incapability of correcting defects, can be
produced at approximately the same cost as that of ordinary photomasks. In
addition, it is possible to prevent charge-up during electron beam
exposure. Further, it becomes possible to control the transmittance and
reflectivity by combination of film thicknesses. In addition, an optimal
multilayer structure can be realized for each of at least two different
kinds of exposure light by controlling the thickness of each layer with
the film composition of each layer maintained as it is.
In the halftone phase shift photomask and halftone phase shift photomask
blank according to the third aspect of the present invention, the
semitransparent layer of the halftone phase shift photomask is made of a
chromium compound which is formed by physical vapor deposition.
Accordingly, a physical cleaning process, e.g., brush cleaning,
high-pressure water cleaning, ultrasonic cleaning, etc., which has
heretofore been used for cleaning photomasks, can be used therefor as it
is. It is therefore possible to produce a clean halftone phase shift
photomask having quality equal or close to that of ordinary photomasks and
also possible to obtain a clean halftone phase shift photomask blank. In
addition, since the semitransparent layer of the halftone phase shift
photomask has a thickness not larger than a half of the thickness of the
conventional semitransparent layer formed by using coating glass, it is
possible to produce a highly accurate halftone phase shift photomask
having even more vertical processed sections.
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